Threaded fasteners are useful in a wide variety of applications where load bearing conditions are required and where high vibrations can result in the loosening or release of fasteners that are not self-locking. Most conventional self-locking fasteners require deformation of the mating nut with a special tool to obtain the necessary mechanical lock. This process, however, can remove the protective finish from the nut, allowing corrosion to take place. Furthermore, a special nut or collar must be used.
Other fasteners achieve self-locking by slitting or displacing the crest of the thread. For example, in U.S. Pat. No. 2,177,005 to Purtell, a set screw is described wherein the crest of the thread is slit, forming a resilient lip that yields upon tightening of the screw, but which resists loosening of the screw by pressing tightly against the upper side face of the female thread. The lip may also be provided with a sharp edge that digs deeply into the side face of the female thread upon reversal of the screw. This type of fastener, however, does not result in adequate locking in certain instances. In one embodiment, the fastener relies on friction alone to prevent loosening. In the alternative embodiment, a mechanical lock is created upon loosening of the screw. This, however, results in the release of at least part of the preload that has been placed on the fastened connection.
A similar fastener is shown in U.S. Pat. No. 3,124,188 to Muenchinger which describes a screw having a helical thread with disaligned segments. The patent states that the screw may be tightened with a minimum of additional resistance caused by the disaligned segments, whereas, when torque is applied in the opposite sense, to remove the screw, the upper or leading end of each segment will tend to bite into the abutting flange of the female thread with which it is associated, thus resisting removal of the screw. The patent recommends case hardening or other treatment of the screw to impart resilience to the disaligned segments. Accordingly, this fastener also suffers from the disadvantages noted above, namely, it locks upon loosening of the screw, permitting a release of the preload intended to be applied to the fastened joint.
It should, therefore, be appreciated that there still is a need for a threaded fastener that achieves a mechanical lock, without loss of preload, and without the use of special tools or special procedures for achieving such a lock. The present invention satisfies this need.